For example, Japanese Patent Laid-Open Publication No. 2004-348496 (Patent Document 1) discloses a technology in which, in a communications system composed of transponders each including a sensor, an authentication number, and a wireless transmission/reception function and a reader/writer, when the sensing results obtained by the transponders are transferred to the reader/writer, the communication frequency and transmission output on the reader/writer side can be varied. By this means, manufacturing variations of transponders and variations in resonance frequency depending on the surrounding environment can be absorbed, thereby improving the communication stability.
Japanese Patent Laid-Open Publication No. 2005-32256 (Patent Document 2) discloses a configuration in which IC tag-sensor units are mounted on many target components and devices such as shaft bearings of a belt conveyer in a thermal power station, for example. Japanese Patent Laid-Open Publication No. 2005-115408 (Patent Document 3) discloses a technology in which passive wireless tags are used as a means for detecting abnormal circumstances such as fire, earthquake, water leakage, and traffic accidents.